Ground or _disc signals_ are fixed at the ground-level, and are worked in conjunction with trap points or outlet switches from sidings. In some cases they are worked direct by a connecting-rod from the 323 switches, and serve merely as indicators to show whether the switches are lying for or against an engine passing out of the siding. In other cases they are worked independently from the signal-cabin by a separate lever and wire connection, the interlocking being so arranged that the lever working the switches must be pulled over before the lever working the disc signal can be moved. In one type the disc signal is fixed to a short vertical axis, as shown in Fig. 494, and by means of a cranked arm is made to rotate a quarter of a circle, so as to exhibit either a stop or advance signal according to the position in which the switches are lying. In another type, the lamp is fixed, and the red disc, with a red glass in the centre, is made to assume a horizontal or vertical position by a rod and crank, as shown in Fig. 495. A simple arrangement of rodding and rollers for switch connections is shown in Fig. 496, the number of sets of rodding being determined by the number of connections to be made. Fig. 497 is a rodding compensator, to compensate or adjust for the difference in length of the rodding arising from variations in the temperature. The compensator may be used either vertically or horizontally, according to space or circumstances. Strong wrought-iron or steel cranks of different angles will be required when changing the direction of the rodding, or connecting to switches and facing point-locks. They must be firmly secured to strong timber framework well bedded in the ballast. For cranks working switches and bolt-locks, it is better to use extra long timbers under the rails instead of the ordinary sleepers. Cross-pieces can be bolted to the ends of the long timbers, and the cranks placed practically on the same timbers carrying the permanent way. By this means the rails and cranks can always be maintained in their proper relative positions as to distance, line, and level. Without a large series of diagrams it would be impossible to adequately describe the extent of signalling and interlocking required at large terminal stations and important roadside stations, but one or two simple examples may serve to illustrate the general principles. [Illustration: Fig. 498, 499] Fig. 498 represents the modern grouping of signals considered necessary at an ordinary double-line junction, showing all the signals at their normal or _danger_ position. The numbers marked on each indicate the numbers of the levers in the interlocking frame of the 325 signal-cabin. Four distinct sets of trains have to be dealt with at this class of junction, and the interlocking must be so arranged that when the signals are lowered for the advance of any one train, no conflicting signals can be given to any other train.